The present invention relates to a capacitive pressure sensor for detecting a change in capacitance (change in electrode-to-electrode distance) based on the deformation of a capacitance chamber caused by application of a pressure from the outside, and a method of manufacturing the same.
In a conventional general capacitive pressure sensor, two opposing electrodes are arranged in a capacitance chamber surrounded by a flexible diaphragm and a base. With this arrangement, a change in electrode-to-electrode distance based on the elastic deformation of the diaphragm is detected as a change in capacitance, so that an external pressure applied to the diaphragm is measured.
WO96/27123 (reference 1) proposes a pressure sensor based on an idea different from a conventional one. In the pressure sensor proposed by reference 1, a capacitance chamber is formed in an elastic substrate made of sapphire or the like, and two opposing electrodes are arranged in the capacitance chamber. With this arrangement, the substrate itself constituting the capacitance chamber is compressed to deform upon application of a pressure. A change in electrode-to-electrode distance accompanying the deformation of the substrate is detected as a change in capacitance.
FIG. 9 shows the pressure sensor disclosed in reference 1. As shown in FIG. 9, this pressure sensor is fabricated by bonding a sapphire substrate 101 with a recess where a lower electrode 101a and lead wire 101b are formed and a sapphire substrate 102 with a recess where an upper electrode 102a and lead wire 102b are formed, such that the electrodes 101a and 102a oppose each other.
The upper electrode 102a is formed on the substrate 102 thicker than a diaphragm, as shown in FIG. 10. When an external pressure is applied, the substrate 102 does not substantially deform, but a substrate portion (to be referred to as a pressure-sensing frame 104 hereinafter) constituting the side wall around the capacitance chamber is compressed to deform. When the pressure-sensing frame 104 deforms, the electrode-to-electrode distance changes, and a change in capacitance is detected. The Young""s modulus of sapphire is 30,000 kg/mm2.
Accordingly, in this pressure sensor, when a pressure is applied, the lower electrode 101a and upper electrode 102a move close to each other while being parallel to each other, and advantages are obtained, e.g., the linearity of the sensor output is maintained. The pressure-sensing frame 104 is not easily compressed when compared to the diaphragm. Hence, this pressure sensor can be applied to a use under a high pressure, e.g., pressure measurement in an injection molder.
In the conventional example, however, when the electrode-to-electrode distance is decreased in an attempt to improve the measurement sensitivity, the pressure-sensing frame 104 is decreased in its size in the longitudinal direction and is not compressed easily, which undesirably decreases the measurement sensitivity. Therefore, the conventional structure can be applied to only a use under a high pressure, and has a very narrow measurement range.
It is an object of the present invention to provide a capacitive pressure sensor in which the measurement sensitivity is improved.
It is another object of the present invention to provide a capacitive pressure sensor in which the measurement range can be widened.
In order to achieve the above objects, a capacitive pressure sensor according to the present invention comprises a first substrate, a first flat electrode formed on the first substrate, a pressure-sensing frame for surrounding the first flat electrode formed on the first substrate, a second substrate bonded to the pressure-sensing frame and opposed to the first substrate, to form a capacitance chamber together with the first substrate and the pressure-sensing frame, a stage formed on the second substrate in the capacitance chamber, separated from the pressure-sensing frame, and opposed to and separated from the first flat electrode, and a second flat electrode formed on the stage and opposed to the first flat electrode, wherein the pressure-sensing frame elastically deforms according to a pressure applied to the first and second substrates.
A method of manufacturing a capacitive pressure sensor according to the present invention comprises the steps of forming a recess in a first substrate and then forming a first flat electrode on the recess, forming a groove having a predetermined depth in one surface of an elastic second substrate to extend circumferentially, bonding a third substrate to one surface of the second substrate directly, polishing the other surface of the second substrate to expose the groove, thus forming a stage surrounded by the groove and forming a second flat electrode on the stage, and bonding the first substrate and a pair structure of the second and third substrates to each other directly, to form a capacitance chamber where the first and second flat electrodes oppose.
With this arrangement, according to the present invention, the length of the pressure-sensing frame which elastically deforms upon application of a pressure can be maintained sufficiently large. Since the upper electrode is formed on the stage, the distance between the upper and lower electrodes can be decreased. Thus, the measurement sensitivity and the measurement range can be further improved.